Graphene-like iron made in the lab

Atom-thick layers of iron have been made in the tiny holes of a perforated piece of free-standing graphene. The work was done by an international team, which has also done calculations that suggest the new material has some potentially useful exotic properties, such as a large magnetic moment. However, the team believes the 2D structure becomes thermodynamically unstable when it is more than 12 atoms wide: a problem that would have to be overcome before the material could be put to work in practical applications such as magnetic data storage.

At first glance, a free-standing 2D metal seems impossible. This is because the bonding between atoms in a metal is mediated by conduction electrons, which are free to move in any direction. As a result, metals tend to have 3D crystal structures and no tendency to form planar sheets. This is unlike crystalline carbon, which is held together by highly directional covalent bonds that allow free-standing atom-thick sheets of graphene to exist. While single epitaxial layers of metal atoms can be created on a substrate, these are not true 2D materials because the atoms are bonded to the underlying structure.

Plugging a gap

In the new research, Mark Rümmeli and colleagues at the Leibniz Institute for Solid State and Materials Research in Dresden, Germany, and at institutes in Poland and Korea studied the behaviour of metal atoms at the edges of holes in graphene. They grew a sheet of graphene by chemical vapour deposition on a surface and detached it by etching the substrate with an iron-chloride solution. This left trace amounts of iron on the surface of the graphene. Irradiating the graphene with an electron beam created small holes and also encouraged the iron atoms to move around. The edge atoms of graphene are the most reactive because they contain dangling bonds; so when the mobile iron atoms encounter the edge of a hole, they bond to it. This continues with iron atoms bonding to the other iron atoms around the edge, until the hole is completely sealed with a 2D square lattice of iron.

The group's theoretical calculations show that the largest thermodynamically stable sheet would be about 12 atoms across – or just 3 nm – wide. The largest sheets observed in the experiment were only 10 atoms wide. Beyond this, the tendency of iron to form a 3D structure wins out over the bonding between the iron and carbon atoms at the edges. "The atoms usually form a tiny crystal that sticks to one of the edges," explains Rümmeli, who is now at the Institute for Basic Science in Korea.

History has shown that when someone comes up with an unexpected new material, someone else usually comes up with an unexpected use for it Mark Rümmeli, Institute for Basic Science, Korea

Other calculations suggest that changes in the electronic band structure of the iron when it forms a 2D lattice should give it a substantially larger magnetic moment than bulk iron. This, the researchers speculate, could make it useful for magnetic memories. Rümmeli stresses, however, that more basic science must be done before the membranes could be considered for practical applications. "History has shown that when someone comes up with an unexpected new material," he says, "someone else usually comes up with an unexpected use for it." The team plans to try to make other 2D metals by the same method and investigate their properties.

Stability problems

Pietro Gambardella, a materials scientist at ETH Zurich, says the work is very interesting from a fundamental perspective, but he remains sceptical of potential engineering applications. "If you have something that breaks down when it becomes larger than 12 atoms across, it's clearly quite unstable," he says, "so it's difficult to see how you could use it in a device without it breaking down."

Arkady Krasheninnikov, an electronic-structure theorist at Aalto University and the University of Helsinki, both in Finland, is more optimistic. "At present, it's clearly too unstable to be useful outside the laboratory," he says, "but it's quite astounding that such a 2D structure can even exist. Now people can start looking for ways to make it more stable." He suggests that it might potentially be stabilized by sandwiching it between two layers of graphene. "Hopefully, it will keep its peculiar magnetic properties," he says.

2-D spin glass

It's interesting that the iron forms a square lattice. Naively, I would have thought it would assume a close-packed hexagonal structure that's more in line with the graphene lattice.

My guess is that there is ferromagnetic frustration going on here. Maybe the 2-D iron lattice minimizes its energy with a square configuration. In such an arrangement the spins would be canted from the easy axis resulting in a strong ferromagnetic interaction along this axis and a weak antiferromagnetic interaction along the orthogonal axis, like a 2-D spin glass.

It's interesting that the iron forms a square lattice. Naively, I would have thought it would assume a close-packed hexagonal structure that's more in line with the graphene lattice.

My guess is that there is ferromagnetic frustration going on here. Maybe the 2-D iron lattice minimizes its energy with a square configuration. In such an arrangement the spins would be canted from the easy axis resulting in a strong ferromagnetic interaction along this axis and a weak antiferromagnetic interaction along the orthogonal axis, like a 2-D spin glass.

It might also be dominated by a valence bond model (ex: sp3 hybridization and resonance bonds for graphene). Some generally unstable s-d hybridization and resonance bonds for iron that only presents itself for small 2-D clusters and later buckles in for larger clusters due to ferromagnetic interactions. I do agree it has to lead to some weird magnetic properties. These small dimensional/volume structures are soo fascinating :p *nerdgazm

"....Other calculations suggest that changes in the electronic band structure of the iron when it forms a 2D lattice should give it a substantially larger magnetic moment than bulk iron. This, the researchers speculate, could make it useful for magnetic memories. ...”

Although this link doesn't mention this idea, the above quote makes me wonder if this could be used to make a material for cheap iron magnets with no rare chemical elements (unlike some magnets ) but with much stronger magnetic fields than normal iron magnets?

If so, perhaps such magnets can one day be made to have a greater magnetic field strength to mass ratio than neodymium magnets ( which are currently the best magnets but a bit expensive ) ?